Silicon nitride based ceramics are considered useful for many applications, such as turbine engines, where it is desirable to achieve higher operating temperatures and efficiencies. Silicon nitride exhibits excellent high temperature properties such as high strength, creep and oxidation resistance and low thermal expansion. Several techniques are known for producing silicon nitride powder such as nitriding of silicon powder; chemical vapor deposition of silicon tetrachloride or chlorosilanes in the presence of ammonia or an H2—N2 mixture; carbothermic reduction of silicon oxide in the presence of nitrogen; and precipitation and thermal decomposition of silicon diimide. The properties of the products from these reactions are dependent on the synthetic route used to produce them. Factors such as metallic and nonmetallic impurity content, shape, morphology and crystallinity of the particles, as well as specific surface area are important in determining the sinterability of the particles, mechanical properties and oxidation resistance. In order to achieve the correct balance of properties, various routes have been explored to manufacture fine, high purity silicon nitride powders that result in increased cost and, in some instances, low yields of product. Therefore, one of the challenges in producing silicon nitride powders lies in finding a cost-effective manufacturing process for high purity, fine silicon nitride powders.
Prochazka et al. (U.S. Pat. No. 4,122,155) describe a method of producing ultrafine silicon nitride powder of high purity. The method employs a gas mixture of silane and ammonia which is heated at temperatures between about 600° C. and 1000° C. producing an amorphous powdery reaction product. The reaction product is heated at a calcination temperature of at least 1100° C. to yield ultrafine silicon nitride powder of high purity. In particular, it is emphasized that the reaction product as well as the silicon nitride powder resulting therefrom should be free of metallic impurities. The silane is of high purity, i.e., at least about 99.9% pure or higher and anhydrous ammonia is used. These requirements simply add to the cost of the manufacturing process.
Inoue et al. (U.S. Pat. No. 4,264,565) and Iwai et al. (U.S. Pat. No. 4,405,589) disclose a process for producing silicon nitride powders from silicon halide precursors by reacting the precursors with ammonia and, then, calcining the resultant reaction product at temperatures ranging from 1200° C.-1700° C., under an inert or reducing gas atmosphere. These processes are complex and have certain disadvantages. For example, the reaction of silicon halide and ammonia is highly exothermic requiring significant cooling to form the intermediate silicon diimide product. The process also requires washing with liquid ammonia to remove ammonium halide byproducts and may contain carbon impurities in the silicon nitride product. The process also requires a final crystallization step of the amorphous product obtained from imide decomposition.
Judin et al. (U.S. Pat. No. 5,178,847) disclose a process for the production of silicon nitride or silicon carbide ceramic whiskers and silicon nitride or silicon carbide ceramic powder from silicon fluoride and ammonia or a hydrocarbon at an elevated temperature. The hydrocarbon or ammonia is decomposed separately at a high temperature into reactive carbon or nitrogen and hydrogen, whereafter the carbon or nitrogen radical thus obtained is further in a gas phase contacted with reactive silicon formed therein from silicon difluoride, in order to deposit finely-divided silicon nitride or silicon carbide out from the gas phase. This process requires the use of a highly reactive difluoride product as the silicon carrier and as the silicon source in the synthesis reaction. The silicon difluoride is formed by reacting silicon tetrafluoride with silicon. The reaction product decomposes to form silicon tetrafluoride which is recycled. Therefore, the silicon tetrafluoride is not the source of the silicon nor is it truly consumed during the reaction process.
Bachelard et al. (U.S. Pat. No. 5,378,666) describe a process for producing whisker-free silicon nitride particulates by carbonitriding silicon dioxide. In particular, silicon nitride spheres, beads, or a variety of other shaped articles exhibiting a regular and controlled particle size, are produced by incorporating a primary reaction mixture of silica and carbon into a porous, carbon-based matrix material. The composite is carbonitrided and excess carbon is eliminated from the carbonitrided composite leaving behind silicon nitride. However, this process requires the addition of one or more carbon blacks and may require the addition of an initial or seed crystallization charge, such as a powder of silicon nitride.
Bachelard et al. (U.S. Pat. No. 5,662,875) describe a process for the manufacture of a fine powder of silicon nitride. The process comprises the reaction, in a nitrogen countercurrent and in continuous fashion, of silica, carbon and a seed crystal, in the presence of a volatile compound of a metal chosen from the group consisting of: Be, Mg, Ca, Sr, Ge, Sn, Ti, Hf, Na and Ba. The reaction zone possesses a temperature gradient, comprising a hot zone in which the metal compound passes into the gaseous state and a cold zone in which the metal compound in the gaseous state condenses. The metal compound in the gaseous state is carried from the hot zone to the cold zone by the nitrogen countercurrent. The metal is added to the reaction to control the particle morphology and size as well as aiding in the subsequent sintering of the powder. However, the addition of metals is undesirable because it causes degradation of the oxidation resistance and mechanical properties of the resulting silicon nitride particles.
Nakada et al. (U.S. Pat. No. 5,470,421) describe a method for purifying an etching solution consisting of an aqueous phosphoric acid solution which has been used in etching of a silicon nitride film. In the process, hydrogen fluoride is added to an etching solution consisting of an aqueous phosphoric acid solution which has been used for etching of a silicon nitride film. The resulting solution is heated to remove fluorides of silicon as reaction products of hydrogen fluoride with silicon compounds which have been contained in the etching solution together with vaporized water. When the silicon compounds as reaction products resulting from the etching precipitate to cause clogging of a filter, the precipitates are removed in the same manner as above. This process enables removal of silicon compounds as reaction products to prolong the life of the etching solution. This process emphasizes the recycling of a starting solution containing phosphoric acid, moreover, there is no mention of recovery of silicon nitride compounds.
An object of the present invention is to provide a process for producing silicon nitride from a starting solution containing fluorosilicic acid derived from a silicon etching process.
Another object of the invention is to recover the waste hydrofluoric acid from the process of the present invention for reuse in the process of the instant invention in addition to producing a silicon nitride compound.